Communication systems using single photon detectors are usually, but not limited to quantum communication systems. For quantum communication systems, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarisation, phase or energy/time and are termed quantum signals. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
An example of a quantum communication is quantum key distribution (QKD) which results in the sharing of cryptographic keys between two parties; a transmitter, often referred to as “Alice”, and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve). In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information.
For successful operation of communication systems employing gated single photon detectors, there is a need to synchronize the transmitter and the receiver of the communication system.
In classical communication, synchronization is straightforward as the (classical) signals that are sent from the transmitter are of strong intensity and consequently regularly detected by the receiver. As such the regularity of the received signals can be used to regenerate the original transmitter clock at the receiver via standard clock recovery techniques. In this way, the transmitter and receiver operate at the same clock frequency allowing accurate time slot assignment of the received signals. However, in communication with single photons, the single photon (quantum) signals that are transmitted are of extremely weak intensity. Due to attenuation and single photon detector inefficiency the number of quantum signals detected by the receiver is much less than what was originally transmitted. The sparseness of the detected quantum signals means standard classical clock recovery techniques are not possible.